The power reserve indicator of a mechanical watch, powered by one or more going barrels, is a complication that is useful for the user.
It is, however, rare, since it often requires considerable modification of the basic movement, and it takes up a great deal of space, in terms of both volume and thickness. This complication is therefore often reserved for movements housed inside quite spacious cases, and it is difficult to broaden its use.
EP Patent EP1970778 in the name of MONTRES BREGUET SA describes a power reserve indicator including a planetary differential gear of quite complex design, of which a first input is connected to the barrel arbor, a second input to the barrel, and an output to a power reserve indicator. The differential gear frame is coaxial with the barrel arbor, and the second input is integral with the barrel cover. The output is a transmission wheel with an inner toothing, which is kinematically connected to the first input by planetary wheels in mesh with each other, and which are carried by the cover.
DE Patent 1015747 in the name of FELSA AG describes a similar differential gear, with a first input connected to the barrel arbor by means of a gear train, a second input connected to the barrel, and a differential output at an output wheel which includes housings for ball or roller bearings inserted between the barrel and the last wheel of the gear train.